1. Field of the Invention
The present invention relates to a magnetic tape in which a servo signal is written onto a DC-magnetized servo band, and a servo writer which subjects a servo band of a magnetic tape to DC magnetization before writing a servo signal thereonto.
2. Description of the Related Art
In recent years, high density recording design in magnetic tapes has advanced, and some magnetic tapes for backup media of computers have a recording capacity of several hundreds of gigabytes. Magnetic tapes therefore have several hundreds of data tracks along the width thereof. Such high density recording design involves excessive narrowing of the data tracks and of the intervals between the adjacent data tracks in a magnetic tape.
Thus, in order to allow recording/reproducing devices of a magnetic head to trace such narrow data tracks, servo signals are written onto a magnetic tape in advance, and the servo signals are then read by a magnetic head, while the position of the magnetic head relative to the magnetic tape (the position along the width of the magnetic tape) is servo-controlled (refer to Japanese Unexamined Patent Application (KOKAI) Heisei No. 8-30942 (Paragraph No. 0016 and FIG. 1)).
The above servo signal is written onto the servo band of a magnetic tape, by applying a recording current to a magnetic head of a servo writer so as to magnetize the non-magnetized servo band in one direction. Conventionally, as shown in FIG. 7A, the servo signals SS are recorded onto the corresponding non-magnetized servo bands SB, by feeding to a magnetic head a recording pulse current PC constituted of a zero current and a plus pulse current, as a recording current. Now that the saturation phenomenon of a servo signal read device such as MR element is avoided by writing the servo signal SS in such a manner.
In the recording pulse current PC, as shown in FIG. 7B, when the zero current out of the recording pulse current PC is fed to the magnetic head, the servo bands SP are not magnetized. On the other hand, when the plus pulse current is fed thereto, due to leakage flux generated from the head gap of the magnetic head, the servo bands SB are magnetized in one direction to form servo patterns SP, so that the servo signals SS are written onto a magnetic tape MT′. Here, each space between the adjacent servo bands SB serves as a data band DB onto which data signals are to be written.
A magnetic tape recording/reproducing device has at least one servo signal read device, such as MR element which changes its electrical resistance in accordance with a magnetic quantity. The magnetic tape recording/reproducing device detects, based on the variation of electrical resistance of the above MR element, the variation of the magnetic field generated from the servo signal SS, and outputs the variation of the magnetic field as a read signal in a differential waveform mode (voltage value). As the variation of electrical resistance of the MR element is increased, the peak voltage value obtained by reading the servo signal SS is increased. As a result, the output read from the servo signal SS has an improved S/N ratio. Accordingly, when the magnetic field generated from the servo signal SS has a great variation, or when the servo signal read device (the MR element) has a large sensing area due to its wide dimensions, the output RSL read from the servo signal SS is increased as shown in FIG. 7(c).
In future, however, it is expected that magnetic tapes will advance to have a memory capacity of several tens of terabytes. As such high density recording design proceeds, the number of data tracks formed on a magnetic tape is increased, the wide of data tracks and interval between adjacent data tracks are further narrowed, and a magnetic tape itself is thinned. Accompanied with this, magnetic quantity which can be detected from the servo signals SS on a magnetic tape is decreased. Further, the variation of the magnetic quantity which can be detected from the servo signals SS by the servo signal read device of a magnetic head is also decreased. Due to this decrease, the output RSS read from the servo signal SS has a low peak voltage value as shown in FIG. 7D, in other words, the S/N ratio of the output RSS is deteriorated. Finally, the magnetic tape recording/reproducing device cannot read the servo signals SS correctly, whereby the position of the magnetic head is unable to be controlled with high precision. For this reason, the deterioration of the S/N ratio has been becoming more serious problem than that regarding the above saturation phenomenon of the MR element.
In view of the above problem, a following technique is devised in the invention already filed by this applicant [JP2003-110396 (not published, that is, not prior art)]. The servo band SB of a magnetic tape MT′ is magnetized by use of a DC erase head (not shown) in one direction (i.e., a DC magnetization) along the length of the magnetic tape MT′, and the servo signal SS is then recorded onto the servo head by magnetizing it in the direction opposite to the one direction (refer to FIG. 8A). Incidentally, in FIG. 8A, the magnetized directions are denoted by small arrows. The output (peak voltage value) which is read from the servo signal SS by the servo signal read device depends on a variation ratio or amount at a boundary between portions onto which a signal is not recorded and onto which a signal is recorded. For this reason, an orientation of a magnetic field greatly changes from the forward to reverse directions at the boundary between the forwardly magnetized portion and the reversely magnetized portion on the servo band. Similarly, the orientation of a magnetic field greatly changes from the reverse to forward directions at the boundary between the reversely magnetized portion and the forwardly magnetized portion. In such a manner, obtained can be, the large output read from the servo signal SS in accordance with such large variation ratio or amount, as shown in FIG. 8B. In other words, the output can have the improved S/N ratio. Here, it can also be considered that applying the above DC magnetization technique to the data band DB improves the S/N ratio of the output read from the data. It has however been recognized that this application yields an undesired result that “DC noise” occurs upon reading of the data, causing the reduction in the S/N ratio.
The above-described technique has had a disadvantage that, if a DC-magnetized region by the DC erase head (hereinafter, refers to as “DC written region”) is largely shifted from the servo band SB and is entered into a part of the data band DB, then “DC noise” occurs upon reproducing of the data signal, causing the reduction in the S/N ratio. Further, if the data are recorded onto the data band aside from this part, then the recording density is decreased by the area of this part. This has caused another disadvantage.
Similarly, when the magnetic tape MT′ is moved along the width thereof after undergoing the DC-magnetization by the DC erase head, the servo signal SS written by the servo signal write head is largely shifted from the DC written region, thereby causing the reduction in the S/N ratio of the shifted servo band.
In consideration of the above disadvantages, the present invention is made. An object of the present invention is to provide a magnetic tape and a servo writer capable of preventing the reduction in the S/N ratio of the output read from the data and servo signals, as well as contributing to the high density recording design.